In a wireless communication system, ubiquitous coverage is the prime requirement for a cellular operator. The wireless communication system primarily includes a homogenous and heterogeneous deployment of cells. In a homogeneous deployment, there will be uniform cell size or coverage area, whereas in case of a heterogeneous network the cell sizes vary according to the deployment of different types of cells. A heterogeneous deployment topology comprises mixed cells served by high power macro eNodeBs (eNBs) and low power pico or femto eNBs or relay nodes within a geographical area.
In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as User Equipments (UEs).
In the LTE system, the user equipment (UE) may perform measurements to measure radio link quality, such as quality of a frequency channel or strength of a radio signal, in order to facilitate handover due to mobility of the UE. The measurement can be divided into two types according to the current operating frequency of the UE, which are an intra-frequency measurement and an inter-frequency/inter-RAT measurement.
The intra-frequency measurement is predominantly performed for the mobility within the same frequency channel (between cells operating on the same carrier frequency); whereas the inter-frequency/inter-Radio Access Technology (RAT) measurement is predominantly performed for the mobility between different frequency channels (i.e. between cells operating on different carrier frequencies).
In addition, the inter-frequency/inter-RAT measurement is performed during measurement gap configured by the network. Further, during the measurement gap, both the uplink and downlink transmissions are suspended (for example, no Physical Uplink Control Channel (PUCCH), physical uplink shared channel (PUSCH), physical downlink control channel (PDCCH), and physical downlink shared channel (PDSCH) transmissions), and thereby the inter-frequency/inter-RAT measurement can be performed within the measurement gap.
Inter-frequency neighbor cell search (measurement) is performed within a gap length of 6 ms in LTE for both Time Domain Duplex (TDD) and Frequency Domain Duplex (FDD) systems. However, some time is required for switching between the frequency of serving cell and the frequency on which a new cell need to be detected. The remaining time of the gap (slightly less than 6 ms) can be utilized for neighbor cell search on one or more configured frequencies, other than the serving frequency. Hence within one gap duration, the actual time for neighbor cell search will be less than 6 ms.
The inter-frequency heterogeneous network deployment may be asynchronous such that radio frame timing of one cell is not aligned with that of the other neighbor cell(s). The measurement gap is standardized in the LTE specification (TS 36.331) and has a periodicity of either 40 ms or 80 ms.
Operators for maximizing offloading opportunity, typically configure a continuous scanning of pico cell frequencies for any possible offloading opportunity. This enormously increases the amount of time that UEs spend in a scan mode.
Further, in a heterogeneous network scenario where small cells are deployed for the purpose of offloading users from macro cell, the constant use of inter-frequency measurements using the current standardized measurement gap patterns and measurement rules will drain the UE battery rapidly. The battery drain is proportional to the Pico cell deployment density within the macro cells.
The currently defined gap pattern was originally designed for mobility purposes within a macro cell only deployment scenario which was quite sparse when compared to the heterogeneous deployments. The measurement gap is designed and optimized for allowing fast inter-frequency/RAT cell search and enabling fast mobility, when needed in a macro cell only deployment scenario. Furthermore, in a macro only network the use of inter-frequency measurements is done when needed by network either for offloading traffic to a different frequency layer or due to mobility reasons.